Along with the recent world-wide tendency toward exhaustion of petroleum resources, general attention is given to shale oil available from oil shale existent underground in large quantities as a new fuel oil. However, since shale oil is contained in oil shale in only such a small amount as for example about 60 kg per ton of oil shale, industrial distillation of shale oil from oil shale requires huge distilling facilities. There is therefore a demand for developing a method for efficiently distilling shale oil from oil shale.
The methods for distilling shale oil from oil shale now in operation on an industrial scale or considered to be in operation on the future in an industrial scale may be broadly classified into the following three types, depending upon the means for supplying the heat for distillation:
(1) with the heat of fuel burning in the distilling furnace; PA1 (2) with the heat of a high-temperature gaseous heat medium blown into the distilling furnace; and, PA1 (3) with the heat of a high-temperature granular heat medium supplied into the distilling furnace.
In the method (1), a fuel and the air are blown directly into the distilling furnace fed with an oil shale to cause combustion of the blown fuel, and shale oil is distilled from the oil shale with the combustion heat of the fuel. This method, having the advantage of a high thermal efficiency for distillation, is problematic in that the quality of the resultant shale oil is degraded by the combustion gas of the fuel and the yield of shale oil is rather low.
In the method (2), a high-temperature gas as the heat medium is blown into the distilling furnace fed with an oil shale, and shale oil is distilled from the oil shale by the heat of the blown gas. This method, being advantageous in the availability of a high-quality shale oil and a high yield, is problematic in that distillation requires a large quantity of high-temperature gaseous heat medium, and an enormous amount of fuel and power costs is required for heating the gas to produce this high-temperature gaseous heat medium and for cooling the gas after distillation for use in recycle.
In the method (3), an oil shale is supplied together with a high-temperature granular heat medium, and shale oil is distilled from the oil shale with the heat contained in the granular heat medium. This method is advantageous in that a high-quality shale oil and a high yield are available and the method requires only small distilling facilities as compared with the method (2). The method (3) is however problematic in that distillation requires a large quantity of high-temperature granular heat medium, and it is not easy to separate the granular heat medium from the waste oil shale after distillation treatment.
Now, the TOSCO II process and the LURGI-RUHRGAS process, which are typical processes representing the method (3), are described below. The TOSCO II process employes, for example, ceramic balls having a diameter of 12.7 mm as the granular heat medium. This process comprises heating ceramic balls to a temperature of from 600.degree. to 650.degree. C. in a heating furnace, and supplying the high-temperature ceramic balls thus heated, together with an oil shale previously dried and then preheated to a temperature of 110.degree. C., into a rotary kiln type distilling furnace; bringing, in the distilling furnace, the ceramic balls into contact with the oil shale; separating from the oil shale a gas containing a gaseous shale oil, hydrogen and carbon monoxide; and, introducing the gas thus obtained from the distilling furnace into separator where said gas is cooled to separate a liquid shale oil from said gas.
The TOSCO II process further comprises discharging the waste oil shale after the separation of the gas and the ceramic balls; separating, by means of a screen, the ceramic balls from the waste oil shale; then, feeding back the ceramic balls cooled through heat exchange with said oil shale thus separated into the heating furnace to reheat it to said prescribed temperature; supplying the fed-back ceramic balls to the distilling furnace to use in recycle; and rejecting the waste oil shale after cooling.
In the process mentioned above, however, since the waste oil shale after separation of the gas is separated from the ceramic balls through a screen, it is necessary to previously crush the oil shale to a size smaller than the diameter of the ceramic balls (12.7 mm for example). This requires special crushing facilities and a crushing process. The quantity of required ceramic balls to be used in recycle is from three to four times that of oil shale fed to the distilling furnace. Furthermore, ceramic balls worn out during use and reduced in size to a diameter smaller than the waste oil shale after distillation cannot be separated through a screen and must be rejected together with the waste oil shale, thus leading to an economic disadvantage.
The LURGI-RUHRGAS process employs waste oil shale as the granular heat medium and comprises heating this waste oil shale to a prescribed temperature in a heating furnace, bringing the waste oil shale into contact with an oil shale in a distilling furnace as in the aforementioned process, and separating by vaporization a gas containing gaseous shale oil from the oil shale through heat exchange with said waste oil shale.
In the above-mentioned process, however, carbon remaining in the waste oil shale after separation of the gas is burnt to utilize the heat of the exhaust gas thereof. Since this combustion treatment covers also the waste oil shale as the granular heat medium of which the residual carbon has already been burnt, combustion of the residual carbon in the new waste oil shale cannot be effected sufficiently, thus making it impossible to utilize the heat of exhaust gas produced by combustion of the residual carbon. In order to solve this difficulty, it would be necessary to separate the new waste oil shale after separation of the gas from the waste oil shale serving as the granular heat medium, but this is practically impossible. In addition, this process requires a large quantity of waste oil shale as the granular heat medium.
As described above, the methods comprising supplying an oil shale together with a high-temperature granular heat medium into a distilling furnace, bringing the oil shale into contact with the granular heat medium in the distilling furnace, and distilling a shale oil from the oil shale through heat exchange with the granular heat medium, although having an advantage of the availability of a high-quality shale oil, require a large quantity of granular heat medium, thus resulting in a low treatment efficiency of oil shale. In addition, it is necessary to crush the oil shale to a specific size for the purpose of separating the granular heat medium from the waste oil shale discharged from the distilling furnace, and this separation may become impossible, depending upon the type of the granular heat medium in use.
A conceivable method for reducing the quantity of required granular heat medium used in recycle is to use a temperature higher than the above-mentioned range of from 600.degree. to 650.degree. C. for the granular heat medium supplied to the distilling furnace and thus to supply the high-temperature granular heat medium to the distilling furnace. However, when employing a granular heat medium of such a high temperature, the surfaces of oil shale in contact with this granular heat medium are locally overheated, producing a carbonized film which prevents satisfactory distillation of the oil shale, and this is not desirable because of the resultant decrease in the shale oil yield.
Under such circumstances, there is a strong demand for the development of a method which permits manufacture of a high-quality shale oil at a high efficiency with the use of a small quantity of granular heat medium when supplying an oil shale together with a high-temperature granular heat medium to a distilling furnace, bringing the oil shale into contact with this granular heat medium, and thus distilling a shale oil from the oil shale through heat exchange with said granular heat medium, but such a method has not as yet been proposed.